The second ability checks if Grave Scrabbler was cast as a result of its madness ability. It doesn’t matter if the specific cost wasn’t paid (for example, if the cost was reduced due to a effect like Cloud Key).

5/1/2007

When Grave Scrabbler enters the battlefield, its ability triggers only if it was cast with madness. If it triggers, you must choose a target, but you don’t decide whether to actually return the card until the ability resolves.

5/1/2007

You can return a card in a different player’s graveyard to that player’s hand.